1. Field of the Invention
The present invention relates to an ultrasonic transducer. More specifically, the present invention relates to an ultrasonic transducer for use with a vicinity alarm utilizing a Doppler effect, a remote control apparatus for a television receiver, and the like.
2. Description of the Prior Art
Typically an ultrasonic transducer for use in a vicinity alarm utilizing a Doppler effect, a remote control apparatus for a television receiver, and the like employs a composite vibrator including an aluminum or a resin resonator provided at the central portion of one surface of a ceramic bimorph vibrator. Such an ultrasonic transducer is desclosed in U.S. Pat. No. 3,675,053 issued July 4, 1972 and British Pat. No. 1,514,967 issued June 21, 1978, for example. The former referenced patent is of a single peak frequency characteristic and accordingly exhibits a relatively narrow utilizable frequency band. On the other hand, the latter referenced patent is of a double humped frequency characteristic and hence exhibits a relatively wide utilizable frequency band.
The present invention is directed to an improvement in an ultrasonic transducer having a double humped frequency characteristic as shown in the latter referenced British Pat. No. 1,514,967.
FIG. 1 shows a sectional view of an example of a conventional ultrasonic transducer. The transducer shown includes a composite vibrator 1 which comprises a resin resonator 3 in the shape of a frustum of a cone fixed in the vicinity of the center of one surface of a ceramic bimorph resonator 2. The composite vibrator 1 is disposed such that the bimorph vibrator 2 is fixed to an insulating base 4 by means of a cylindrical supporting member 41 formed integrally with the insulating base 4. External connection terminals 91 and 92 are provided through the base 4 and are electrically connected to the corresponding layers of the bimorph vibrator 2 by means of lead wires 2a and 2b, respectively. The base 4 as well as the composite vibrator 1 is covered with a metallic casing 6. The casing 6 is formed with an opening 61 at the top surface thereof for emitting outward of the casing ultrasonic energy generated by the composite vibrator 1 or receiving ultrasonic energy from the environment. The opening 61 is covered with a screen member 7. The screen member 7 is sandwiched in the casing 6 between an edge 62 of the opening of the casing and a ring member 8, with the ring member 8 supported by an annular shelf 63 in the casing 6. The shelf 63 may be formed by protruding inward the peripheral side surface of the casing by a drawing process, for example. A shield plate 5 is provided such that the same is fixed by caulking as at the end 64 of the casing 6. The metallic casing 6 and the shield plate 5 are to electrostatically shield the composite vibrator 1.
FIG. 2 is a graph showing an impedance characteristic typical of the FIG. 1 ultrasonic transducer. FIG. 3 is a graph showing a sensitivity characteristic typical of the FIG. 1 ultrasonic transducer. As seen from FIG. 2, such an ultrasonic transducer as shown in FIG. 1 gives rise to the first resonance region at the lower frequency region exhibiting the first sensitivity and the second resonance region at the higher frequency region exhibiting the second sensitivity. As a result of experimentation by placing powder on the transducing surface of the resin resonator 3, it has been observed that the first resonance region and the second resonance region are based on different vibration modes. More specifically, the first resonance region is observed as vibration of an up and down vibration mode or "a piston vibration mode" wherein powder distributed on the transducing surface is vibrated up and down throughout the whole surface thereof. On the other hand, the second resonance region is observed as vibration of a bending vibration mode, inasmuch as the powder distributed on the transducing surface is concentrated along the nodal line. Nevertheless, as seen from FIG. 3, the sensitivity level at the first resonance region is considerably lower than the sensitivity level at the second resonance region. Referring to FIG. 3, a practically utilizable sensitivity level for such an ultrasonic transducer is shown by broken line A. As seen from FIG. 3, the sensitivity level of the first resonance is not sufficiently large to exceed the practically utilizable sensitivity level A, with the result that an ultrasonic wave can hardly be transduced in the relatively low frequency region with such low sensitivity level. Accordingly, a conventional ultrasonic transducer as shown in FIG. 1 can merely provide a narrow frequency range B as shown in FIG. 3 where only the second resonance region occurs. The low sensitivity level of the first resonance region may be accounted for as follows; since the composite vibrator 1 is directly fixed to the base 4 by means of the cylindrical supporting member 41, the vibration in the piston vibration mode is suppressed, with the result that the sensitivity at the first resonance region is low.